1. Field of the Invention
The present invention relates to an optical illumination system designed to emit light onto a reflective display device with micromirrors that turn light on or off by changing angles of reflection, and a projection system including the same.
2. Description of the Related Art
In general, an image projection system that can be applied to rear projection televisions or front projection projectors includes an optical illumination system, a display device, and a projection optical system. The display device modulates light illuminated by the optical illumination system, and the projection optical system projects the modulated light to display an enlarged image.
Projection systems have been developed to use a digital light processing (DLP) device fabricated using micro electro mechanical system (MEMS) technology as a reflective display device. The DLP device is also referred to as a digital micromirror device (DMD).
A DLP device for creating an image used in a projection system is a two-dimensional array made up of a plurality of micromirrors, each of which represents a single pixel. To create an image, each micromirror is driven in response to a corresponding image signal and turns light on or off by changing a reflection angle of incident light.
When the DLP device is used as a reflective display, an optical system is constructed so that a path of light emitted from an optical illumination system and entering the DLP device is different from that of light reflected by the DLP device in order to produce an image.
Typically, a total reflection prism is used in an optical illumination system to separate light emitted from the optical illumination system from light propagating toward a projection optical system.
U.S. Pat. No. 5,604,624 discloses an optical system for projection display configured such that light emitted from a light source passes through a total reflection prism and is incident on a DLP device (called DMD therein).
In general, a prism type projection system using a total reflection prism to separate light is constructed such that the total reflection prism is positioned in the optical illumination system to totally reflect light and illuminate the light onto a reflective display device. The illuminated light is reflected by the reflective display into a projection optical system. In this case, although the light entering the projection optical system strikes the total reflection prism, most of the light is transmitted toward the projection optical system since an incident angle of light with respect to a surface of the total reflection prism is small. That is, the light illuminated into the reflective display device is separated from light reflected by the reflective display device under a total reflection condition of the prism.
As disclosed in the above-cited reference, the total reflection prism is comprised of two or more prisms to separate the reflected light from light illuminated from the light source. In this case, the two or more prisms are fabricated and assembled in different shapes. Each prism must be fabricated to have a precise angle.
Thus, difficulties in fabrication and assembling the total reflection prism degrade manufacturing and assembling capabilities, thus increasing manufacturing costs. The light reflected off a total reflection prism plane between the reflective display device and the projection optical system tends to enter the projection optical system, which may adversely affect image contrast.